1. Field of the Invention
The invention relates to an internal gear oil pump for a vehicle, and more particularly, technology for reducing rotational resistance of that internal gear oil pump for a vehicle.
2. Description of the Related Art
One known internal gear oil pump for a vehicle is provided with a pump body having a pump chamber formed by a cylindrical inner peripheral surface, an annular driven gear that has internal teeth and is rotatably supported by the cylindrical inner peripheral surface by fitting with the cylindrical inner peripheral surface, and a drive gear that has external teeth that mesh with the internal teeth of the driven gear, is rotationally provided about a rotational center that is offset from the rotational center of the driven gear, and rotatably drives the driven gear. In this kind of internal gear oil pump for a vehicle, when the driven gear is not rotating, its own weight causes it to contact the pump body. However, when the driven gear is being rotatably driven, hydraulic fluid in the annular gap between the driven gear and the pump body is dragged by the rotation of the driven gear, and consequently, moves in the circumferential direction in the gap. As the hydraulic fluid flows into the gradually narrowing gap toward the location where the driven gear and the pump body are close together, maximum dynamic pressure is generated at that location, such that the driven gear is supported without contacting the pump body. Incidentally, this dynamic pressure is pressure that acts to push the outer peripheral surface of the driven gear toward the inner peripheral side.
One problem with this internal gear oil pump for a vehicle is that the driven gear wobbles, that is, the rotational center of the driven gear wobbles, at low rotation speeds and when a large amount of hydraulic pressure is generated, for example. This wobbling of the rotational center of the driven gear may cause the lubrication state between the outer peripheral surface of the driven gear and the cylindrical inner peripheral surface of the pump body to become a boundary lubrication state, such that friction loss occurs which may increase the rotational resistance of the driven gear. To solve this, Japanese Utility Model Application Publication No. 61-171885 (JP-U-61-171885) proposes technology for suppressing wobbling of the rotational center of the driven gear. In JP-U-61-171885, a plurality of concave portions, each having a stepped cross-section orthogonal to the rotational axis of the driven gear, are provided at predetermined intervals in the circumferential direction on the outer peripheral surface of the driven gear. When the driven gear is rotatably driven, far more dynamic pressure than is generated with a structure that lacks the concave portions is generated in the hydraulic fluid in the part of a gap, which is formed between the outer peripheral surface of the driven gear and the pump body, where the concave portions are located. Compared with a structure that lacks the concave portions, the self-aligning ability of the driven gear in the internal gear oil pump for a vehicle described in JP-U-61-171885 is improved due to the far greater dynamic pressure acting on the driven gear, which enables the wobbling of the rotational center of the driven gear to be suppressed.
Incidentally, in the internal gear oil pump described above, the problem of the rotational center of the driven gear wobbling when the driven gear rotates is solved, thus making it possible to maintain a good lubrication state between the driven gear and the pump body, by generating a relatively large amount of dynamic pressure by forming the plurality of concave portions on the outer peripheral surface of the driven gear as described above. As a result, friction loss due to the driven gear and the pump body contacting or being in close proximity to one another is able to be suppressed, so in this respect, rotational friction of the driven gear is considered to be reduced. However, forming the plurality of concave portions results in a pressure drop at the part of the gap between the driven gear and the pump body where the height of the gap increases in the direction opposite the rotational direction of the driven gear, due to peeling at the flow lines of the hydraulic fluid that flows by that part. Therefore, the pressure difference in the circumferential direction of the gap between the driven gear and the pump body increases, causing force that impedes the rotation of the driven gear, i.e., pressure drag (pressure resistance), to act on the driven gear. As a result, a new problem arises in which the rotational resistance of the driven gear increases.